An abnormality in the renin-angiotenisn system has long been suggested as a primary factor in the development and maintenance of hypertension. However, to date no firm correlation has been obtained between renin or angiotensin levels and the presence or severity of hypertension. One factor which occurs with a very high frequency in hypertensive situations is an elevation of plasma levels of angiotensinogen. Angiotensinogen, molecular weight of 57,700, is the precursor protein of angiotensin II, molecular weight of 1300, has been found to occur in multiple forms in pooled hog serum. The formation of angiotensin; then, a dipeptidase removes two amino acids from the decapeptide. This investigation will focus on the contribution of angiotensinogen to the hypertensive state. Emphasis will be placed on a definition of brain angiotensinogen and its relationships to the peripheral (blood) prohormone. The rationale for this emphasis is the demonstration that central pathways of blood pressure regulation may be implicated in the pathogenesis of essential hypertension and we have shown that angiotensinogen is widely distributed throughout the CNS. The experimental methods employed in this study will be applied at a later date to human investigations.